A brassica, firm and large with edible tip (4)
I believe the answer is:
cole
'a brassica' is the definition.
(term for any brassica plant)
'firm and large with edible tip' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'and' means one lot of letters go next to another.
'large' becomes 'l'.
'with' says to put letters next to each other.
'tip' suggests taking the first letters (I've seen 'tips' mean this (tip can refer to the top end of something)).
The first letter of 'edible' is 'e'.
'co'+'l'+'e'='COLE'
(Other definitions for cole that I've seen before include "died in 2015" , "Old English king!" , "Elderly monarch" , "Old nursery rhyme king" , "Cabbage" .)